Balancing of reciprocating engines



. pril .23, 1946.

w. H.' PURDIE 2,398,961

. BALANCIG OF- REGIPROCATING ENGINES .Filed sept. 18, 1945 f2 Sheets-Sheet 1 y w. H. PURDIE' 2,398,96l BALANCING OF RECPROGATING EI-IGINES v April 23, 1946.

Filed sept. 18, 1945 ,moz woz.

Inventor l/l/i/l'am Pu rc1/ie A ttorney Patented Apr. 23, 1946 BALANCING F RECIPROCATING ENGINES William Hamilton Purdie, Sunderland, England, assignor to William Doxford & Sons Limited, Sunderland, England, a British company Application September 18, 1943, Serial No. 502,914 In Great Britain September 22, 1942 3 Claims. (Cl. 123-51) This invention relates to improvements in the balancing of reciprocating engines, and has for its object to provide for balancing the scavengepumps of opposed-piston oil-engines of the multicylinder single-crankshaft vertical type, having an even number of cylinders with one scavenge pump driven direct from the crankshaft and situated at the centre of the group of cylinders. It is particularly applicable to, but not limited to, engines with four or six cylinders.

In opposed-piston engines of this type, each cylinder is served by a group of three cranks of which the middle one is connected to the lower piston in the cylinder, and the two outer cranks, which are 180 from the middle crank, are connected to the upper piston; further it has been customary to arrange the strokes of the upper and lower pistons in inverse proportion to the reciprocating weights so that the primary vertical forces for each individual cylinder are balanced. The secondary vertical forces and couples for the working cylinders are balanced collectively by suitable crank sequence, such' for example, as l-3-2-4 for four-cylinder engines with cranks equally spaced 90 apart, and 1-4-2-6-3-5 for six-cylinder engines having cranks equally spaced 60 apart. The revolving parts of the crankshaft and connecting rods of the working cylindersare so proportioned that there are no unbalanced forces for each individual cylinder, and therefore no collective force or couple for thecomplete engine.

When a single-piston scavenge-pump is tted at the centre of the engine, unbalanced primary and secondary forces are introduced. It is well known that the unbalanced forces due to the revolving parts of such pump, viz.: the connecting rod, the crankpin and crankwebs, can be reduced to zero by the addition of suitable balance- Weights on the crankwebs, but any further reduction of the vertical unbalanced primary force by the adoption of larger balance-weights introduces an unbalanced horizontal force which may be undesirable.

Since the centre crankpins of the pairs of working cylinders situated at equal distances on eith'er side of the scavenging pump are 180 to each other to secure collective balance of the unbalanced secondary couples due to these cylinders, it has been proposed to secure balance of the vscavenging pump by placing the scavenging pump crankpin in line with any one of the centre crankpins, and adjusting the reciprocating weights of two cylinders only, in the following manner. Assuming that the stroke of the scavenge-pump is equal to th'e stroke of the lower pistons the reciprocating weight of the lower piston in phase with the scavenge-piston is reduced by 50% of the reciprocating weight of the scavenge-pump parts, and the reciprocating weight of the cylinder the centre crankpin of which is at to the scavenge-crank is increased by a similar amount; the unbalanced Vertical primary force due to the scavenge piston assembly will thereby be reduced to zero, and no primary unbalanced vertical couple will be introduced.

If the scavenge-pump stroke differs from the lower piston-stroke, the amounts by which the piston reciprocating weights are increased or decreased will require modication `in direct Proportion to the ratio of the scavenge-pump stroke to the lower piston-stroke.

If the reciprocating weight of the scavengepump is considerable, however, the necessary reduction in the reciprocating weight of the one lower piston may be impracticable, and the necessary additional weight required to be added to the other piston may be larger than can be accommodated conveniently-say inside the pistonskirt.. Moreover, the increased weight necessitates heavier lifting-gear for assembly and overhauling purposes, and in a multi-cylinder engine three types of the lower piston assembly would be necessary,viz.: normal, light and heavy, so that additional spare parts are required. Furth'errnore, the heavy out-of-balance forces thereby acting on individual cylinders cause considerable bending stresses in the engine-structure.

According to the present invention, thereciprocating weights of all the lower (or upper) pistons of the working cylinders'are modified, half of the number being increased and the other half decreased, all bythe same amount, and the position of the scavenging crank is arranged relatively to the other cranks so that the unbalanced primary vertica1 and horizontal forces and couples for the complete engine remain at zero. whilst the unbalanced secondary vertical forces 'and couples are reduced to unimportant values. 'i

According to another feature of this invention, the scavenging crankis spaced centrally in a consecutive group of half the total number of cranks, and the lower pistons of said group of cranks are reduced in weight by equal amounts, whichk amounts are such that, taking into account their phase relationship, 'they are jointly equivalent to half the Weight of the scavengepump assembly, and the remaining lower pistons are increased in weight by the same equal amounts. l

In the accompanying drawings:

Figure l is a more or less diagrammatic sectional elevation showing one construction and arrangement of a cylinder unit of one type of engine to which the invention may be applied,

Figure 2 is a purely diagrammatic view illustrating the application of the invention 4toa fourcylinder engine,

Figure 3 is a crank sequence diagram of the engine of Figure 2,

Figure 4 is a diagram showing the application of the invention to a six-cylinder engine, and

Figure 5 is a crank sequence diagram for the engine illustrated in Figure il.

Referring first to Figure l, the reference 'I0 in- 1 dicates a cylinder having a lower piston I I and an upper piston I2 which are reciprocated in opposition to one another. Each such cylinder of the engine is served by a group of three cranks on a crankshaft I3, whereof the middle crank I4 is connected vto the lower piston II and the two outer cranks I5 which are disposed at 180 from the crank I'II, are connected through suitable side rods toa cross-'head I6 connected by the rod I'I to the upper piston I2. The two pistons II, I2 therefore move Yin opposition to one another and they may have equal strokes, or as illustrated, the lower vpiston II may have a longer stroke than that of the upper piston I2. Suitable ports for the inlet of scavenging and combustion air and for the delivery of the exhaust are controlled by the. pistons, and the fuel is injected by a suitable noZzle indicated at I8. The balancing of one cylinder unit of Vthe engine is ordinarily effected by making the reciprocating weights for the two pistons equal, if they have strokes of the same length, or to make the weights in inverse proportion to the strokes if the latter are of diierent length. This description is given merely by way of example of one construction of engine to which the invention may be applied.

lIfurning now to Figure 2, there is illustrated diagrammatioally a four-cylinder engine in which the cylinders arenumbered respectively, No. l, No. 2,"No. 3, and No. 4 and each is provided with two` pistons coupled to a single crankshaft indicated generally by the reference I9.

A reciprocating scavenge-Dump arranged at the middle of the row of cylinders is constituted by a cflinder'ZllI with a piston 2l driven from the crankshaft I9 by a crank 22. Balance weights 23 may be provided for the crank 22 to give revolving balance.

For such a four-cylinder engine, a suitable crank sequence or ring order is 1 3-4-2 as shown in Figure 3, this diagram referring to the centre crank for each cylinder unit. In this particular example, the pistons whereof the weight is modified to eect balancing in accordance with this invention, are the lower pistons of the four cylinders indicated at 24, 25, 26 andVZ'I respectively, and the reciprocating weight of the scavenge-.pump 20, 2| is balanced by reducingv the weights of two of the lower working pistons and increasing the Weight of the other two. For this purpose the crank 22 of the scavenge-pump is set at the vposition SP in Figure 3 mid-way between the centre cranks of No. 3 and No. 4 cylinders, these being two consecutive cranks in the crank sequence diagram. The reciprocating weights of the pistons 26 and 21 for the cranks No. 3 `and No. 4 are reduced, conveniently by tting thinner piston-skirts to those pistons.u or in any other convenient manner, and theV pistons 24 and 25 for No. 1 and No. 2 cylinders have their weight increased by the same amount. Owing to the phase displacement of the cranks, the weight which has to be added to or subtracted from the working pistons to achieve this balance is equal to li\/2=0.353 of the reciprocating weight of the scavenge-piston assembly, when the the lower 'piston-s. If 'its stroke be greater or less than that of the working pistons the amount to be added or subtracted will be greater or less in proportion to the respective strokes.

In Figure 2 therefore the pistons 26 and 2l have their weight reduced by providing thinner -skirts as mentioned above and the pistons 24 and 25 have 'their weights increased by means of suitable masses -28A, -29 which may be secured inside the skirt, or disposed in any other convenient manner. It is a convenience from the point of view Vof carrying spares, that the four pistons 24, 25, 26, 21, should all be identical with one an other and the masses 28l and '29 should be separate and attachable at will, so that only one kind of spare piston need be carried for all the lower working pistons.

Figure 4 is a diagram similar to Figure 2 showing one mode of applying this invention to a sixcylinder engine. The various cylinder units are marked Nos. 1 to 6 and a suitable crank sequence for such an engine is shown in Figure 5, as 1 4- 2-6-3--5- lIn Figure 4 the crankshaft is indicated generally by the Vreference 3i! and the scavenge-pump l3I is situated at the middle of the length of the engine between cylinders No. 3 and No. 4. Its crank 32 is shown as vgiving it a Stroke equal to that of the lower working pistons "of the cylinders and it is arranged, as above described, centrally in 'the consecutive group of cranks 3 5-l, so that 'its angular disposition is the same as that 'of crank No. 5.

In order to effect the balancing of the scavengepurnp, the pistons of this group have their weight reduced and the pistons of the other group 4-2-6 have their weight increased by the `same amount. Owing to the phase relationship of the cranks of the various cylinders,'the amount which i's subtracted from 'each 'of the pistons 3, 5, 1, and the amount which is added to each of the pistons V2, 4, 6, is equal to one quarter of the reciprocating weight of the scavenge-p'ump `assembly when the stroke of the scavenge-pump is the same as that of the lower piston. If the stroke i's greater or less than that of the lower pistons, the amount to be added or subtracted will be kgreater or less in proportion as before. The 'details of construction mentioned above in describing Figure 2 apply equally to the six-cylinder engine and therefore need not be repeated. Y

In both of the above examples, the modification of the weight of the pistons to effect balancing of' the scavenge-pi'sto'n has been described as applied to the lower working pistons lin 'the cylinders, but it will be appreciated that it could equally well be applied tothe upper setjo'f pistons; it will also be appreciated that, if desired, the modification of weight could be applied to both 'the upper and lower working pistons, but 'ordinarily this will not be preferred as it will' increase the out-of-balance forces for'the individual Cylinders.

The advantages o'f this invention are as inentioned above, that elimination ofv the primary vertical and horizontal unbalanced forces-or couples for the complete engine isobtained, together with the reduction ofthe unbalanced secondary vertical forces and couples to unimportant values,

whilst still retaining the working parts of all the cylinders duplicates of one another. Moreover, the modification in weight to increase or decrease the weight of the different parts being distributed A over a large number of cylinders, comes within a practicable range and does not require any substantial increase in the lifting capaci-ty of the overhauling gear, The individual out-of-balance forces for the various cylinders are smaller than with the previously known methods of balancing the scavenge-pump, so that the bending stresses on the engine structure are reduced.

I claim:

1. In an oil engine of the vertical, multi-cylinder, opposed-piston, single crankshaft type, the combination of a plurality of working cylinders, a reciprocating scavenge-pump at the middle of the row of cylinders, a set of upper pistons in said cylinders, a set of lower pistons in said cylinders, whereof the weight of one halfof one set of plstons is increased and the weight of the other half is decreased all by the same amount, and the position of the crank for the scavenge-pump is arranged relatively to the cranks of the said pistons so that the unbalanced primary vertical and horizontal forces and couples for the complete engine are maintained at zero and the secondary vertical forces and couples are reduced to unimportant values. l v

2. In an oil engine of the vertical, multi-cylinder, opposed-piston, single crankshaft type, the combination of a plurality of working cylinders, a set of lower pistons and a set of upper pistons in said cylinders, sets of angularly spaced cranks connected to said sets of pistons respectively, a reciprocating scavenge-pump situated at the middle of the row of cylinders, a separate crank for the scavenge-pump spaced centrally in a consecutive group of half the total number of cranks of one set of pistons, the pistons of said set being reduced in weight by equal amounts, which amounts are such that, taking into account their phase relationship, they are jointly equivalent to half the reciprocating weight of the scavengepump assembly, and the remaining pistons of said set are increased in weight by the same amount.

3. In an oil engine of the vertical, multi-cylinder, opposed-piston, single crankshaft type according to claim l, the combination with the set of pistons whereof the Weights are to be modified, which pistons are all of the minimum weight, of detachable additional weights and means for securing them to said pistons.

WILLIAM HAMILTON PURDIE. 

